1. Field of the Invention
The present invention relates to a four wheeled vehicle such as an automobile or the like which is endowed with a function of changing roll angle control mode according to the magnitude of the side acceleration acting upon the vehicle body, and to a control method for such a vehicle.
2. Description of the Related Art
With a four wheeled vehicle such as an automobile or the like, when the vehicle is running around a curve, due to lateral force such as centrifugal force acting upon the vehicle body, rolling occurs in which the vehicle body tilts towards the outside of the curve, with a difference in the vertical deflection of the left and right suspension devices of the vehicle being set up. As this rolling increases, an increasing reaction force is generated; and stabilizers are per se known as means for preventing the vehicle body from rolling to an excessive extent. Furthermore, as such a stabilizer, there is a per se known type of active stabilizer which is provided with an actuator, and which is made so that its anti-roll force can be variably adjusted. According to this type of active stabilizer, it is possible to adjust the roll rigidity, which is indicative of the anti-roll characteristic of the vehicle body, in a variable manner according to the magnitude of the lateral force which acts upon the vehicle body, and, furthermore, by doing this, it is possible to control the magnitude of the roll angle to which the vehicle body rolls in a variable manner, according to the magnitude of the lateral force which acts upon the vehicle body.
Furthermore, with regard to variable control of the roll rigidity and the roll angle of the vehicle body, if the vehicle wheel suspension devices are provided with active suspension units, like air springs, which are able to control their spring force in a variable manner, then this type of control operation also becomes available.
On the other hand, with regard to the roll rigidity of the vehicle wheel suspension devices, the larger this roll rigidity is, the smaller is the tilting of the vehicle body to the outside of a curve around which it is running, but, along with the rolling of the vehicle body, the greater is this rolling, the more does the ground contact load upon the vehicle wheels shift to the outside of the curve, and the allotment of the ground contact load between the left and right vehicle wheels becomes more greatly biased towards the outside of the curve. Since, as shown in FIG. 6, the increase of the cornering force upon the vehicle wheels with respect to increase of the ground contact load upon the vehicle wheels exhibits a non linear characteristic which curves towards saturation in an upwardly convex shape, accordingly the total cornering force upon the left and right vehicle wheels becomes smaller along with greater bias of the allotment of the ground contact load between the left and right vehicle wheels from a state of 50:50 equilibrium (in the example shown in the figure, through 40:60, 30:70, 20:80).
With a four wheeled vehicle, the relative magnitude relationship between the magnitude of the cornering force upon the front wheels and the magnitude of the cornering force upon the rear wheels affects the steering responsiveness of the vehicle. In other words, when the cornering force upon the front wheels becomes small as correlated with the cornering force upon the rear wheels, the vehicle exhibits an understeering characteristic; but conversely, when the cornering force upon the rear wheels becomes small as correlated with the cornering force upon the front wheels, the vehicle exhibits an oversteering characteristic. Since, according to the above, the cornering force is affected by the roll rigidity, the relative magnitude relationship between the roll rigidity of the front wheel suspension devices and the roll rigidity of the rear wheel suspension devices affects the steering responsiveness of the vehicle. Various methods have been proposed for controlling this relative magnitude relationship between the roll rigidity of the front wheel suspension devices and the roll rigidity of the rear wheel suspension devices, which may be termed the roll rigidity front/rear allotment ratio, in various ways. For example, in Japanese Patent Publication No. JP-A-2-193749, it is described to perform control by establishing a relationship between the roll rigidity ratio between the front wheels and the rear wheels, and the braking force upon the rear wheels.
In Japanese Patent Publication No. JP-A-2-193749, apart from the gist of that invention, in the description of an embodiment, the concept is included of, in connection with control of the above described roll rigidity ratio, also performing control so as to determine the control amount with reference to a target roll angle. However, the desirable target value for the roll angle and the desirable target value for the roll rigidity front/rear ratio do not necessarily match one another, and, as a general rule, it is difficult to perform control while giving serious consideration to both of them at the same time. Furthermore, if both of these parameters are controlled at the same time, there is a danger that large control errors will occur due to interference taking place between them. On the other hand, roll angle control is particularly effective when the vehicle is running around a curve at comparatively low speed, so that the driver has a margin of attention for considering the inclination of the vehicle body. However, when the vehicle is running around a curve at a speed which is higher than a certain level, what is the matter of most concern from the point of view of the driver is the steering responsiveness of the vehicle, i.e. how the vehicle responds to being steered.